This invention relates to communication systems and in particular to systems for modulating and demodulating a communication signal.
The well known methods of modulating a signal of the prior art includes amplitude modulation, frequency modulation, pulse position modulation, pulse width modulation and pulse code modulation.
Amplitude modulation takes a low frequency incoming signal and applies it to a high frequency carrier to vary the amplitude of the carrier in accordance with the amplitude of the incoming signal.
Frequency modulation takes a low frequency incoming signal and converts its variations in amplitude into variations in frequency of a high frequency carrier above and below a given frequency.
Pulse position modulation takes a low frequency incoming signal which is converted into a pulse whose position in a fixed time period corresponds to an instantaneous amplitude of the signal.
Pulse width modulation takes a low frequency incoming signal which is converted into a pulse whose length corresponds to the instantaneous amplitude of the incoming signal.
Pulse code modulation takes a low frequency incoming signal and converts it into a group of pulses defining a binary code corresponding to the instantaneous amplitude of the incoming signal.
Each of the prior art modulation methods can be multiplexed for transmission over a common pair of conductors, however, the amount of information that can be transmitted for a unit period of time is limited either by the frequency of the modulated carrier or the pulse period.
In all of the prior art methods of modulation, the time period of the incoming signal is the same time period for the modulated signal, that is, a given amount of information per unit of time in the modulated signal is the same as for the amount of information in the incoming unmodulated signal.